1. Field of the Invention
The present invention relates to a multi-beam scanning optical system and an image forming apparatus using the same and, more particularly, to a multi-beam scanning optical system suitably used for an image forming apparatus, e.g., a laser beam printer (LBP) or digital copying machine, which can obtain a high-resolution, high-quality image without any print position error in the main scanning direction by properly placing a slit member as a constituent element of a synchronization detection means.
2. Related Background Art
Conventionally, as a method of enabling high-speed optical scanning, a method (multi-beam scanning optical system) of simultaneously scanning a plurality of beams of light (light beams) on a scanned surface and forming a plurality of scanning lines at predetermined intervals on the scanned surface by using a multi-beam light source (multi-laser light source) as a laser light source is known. As multi-laser sources that can be used in such a multi-beam scanning optical system, for example, the following light sources are available:
(1) a light source having a plurality of emission points (light-emitting portions) on one chip;
(2) a light source in which a plurality of laser emission elements are used to combine optical paths by a beam splitter; and
(3) a light source for splitting one light beam into a plurality of light beams by using a beam splitter and independently driving modulators provided for the respective split light beams.
FIG. 5 is a schematic view showing the main part of a conventional multi-beam scanning optical system having two light emission points on one chip.
Referring to FIG. 5, a plurality of light beams optically modulated in accordance with image information and emitted from a multi-beam semiconductor laser 51 serving as a multi-laser source are converted into substantially parallel light beams or convergent beams by a collimator lens 52 and strike a cylindrical lens 53. Of the light beams incident on the cylindrical lens 53, the light beams emerge without any change in a main scanning cross-section but converge in a sub-scanning cross-section to be formed into substantially linear images (linear images elongated in the main scanning direction) on a deflecting surface (reflecting surface) 54a of an optical deflector 54. The plurality of light beams reflected/deflected by the deflecting surface 54a of the optical deflector 54 are formed into spots on a scanned surface 56 by an imaging optical system (f-xcex8 lens system) 55 having first and second f-xcex8 lenses 55a and 55b exhibiting different powers in a sub-scanning cross-section. By rotating the optical deflector 54 in the direction indicated by an arrow A, the light beams are scanned on the scanned surface 56 in the direction indicated by an arrow B (main scanning direction) at a constant speed. Note that FIG. 5 shows only one light beam.
In this multi-beam scanning optical system, to accurately control the write position of an image, a synchronization detection means is generally placed immediately before a position where an image signal is written.
Referring to FIG. 5, a slit member (BD slit) 83 is placed at a position equivalent to the photosensitive drum surface 56. An optical sensor (BD sensor) 84 serves as a synchronization detection element. Note that each of the BD slit 83, BD sensor 84, and the like forms one element of a synchronization detection means 91.
Referring to FIG. 5, the timing at the scanning start position of image recording on the photosensitive drum surface 56 is adjusted by using an output signal from the BD sensor 84.
FIG. 6 is a sectional view showing the main part of the BD slit 83 in FIG. 5 when viewed from the light beam incident side. Referring to FIG. 6, the BD slit 83 has first and second edge portions 83a and 83b. The first and second edge portions 83a and 83b are arranged parallel to the Z-axis in the coordinate system in FIG. 6. First and second laser spots 11 and 12 of a plurality of light beams (BD light beams) for synchronization detection are formed on the BD slit 83 surface. When the optical deflector 54 rotates in the direction indicated by the arrow A in FIG. 5, the first and second laser spots 11 and 12 are respectively scanned in the directions indicated by arrows A3 and A4 in FIG. 6.
As shown in FIG. 6, the first and second laser spots 11 and 12 are spaced apart from each other by predetermined distances in the main scanning direction (Y-axis direction) and sub-scanning direction (Z-axis direction). If the distance in the main scanning direction is represented by Lxe2x80x2, the first and second laser spots 11 and 12 are scanned on the scanned surface 56 while always being spaced apart from each other by the distance Lxe2x80x2 in the main scanning direction at the same time.
A scanning start point 61 (image writing start position) of a plurality of light beams A1 for image formation on the scanned surface 56 is determined as follows.
Assume that BD detection corresponds to the timing at which a BD light beam B3 strikes the BD sensor 84 placed above the scanned surface 56 on the upstream side in the main scanning direction. This BD detection is independently performed for each light beam, and image writing starts a predetermined time delay after the BD detection.
To more accurately detect the timing at which the BD light beam B3 strikes the BD sensor 84, the BD slit 83 is placed in front of the BD sensor 84. As described above, the BD slit 83 is made up of the first and second edge portions 83a and 83b. A distance L between the first and second edge portions 83a and 83b in the main scanning direction is set to be smaller than the distance Lxe2x80x2 between the first and second laser spots 11 and 12 in the main scanning direction. This setting prevents the first and second laser spots 11 and 12 from simultaneously striking the BD sensor 84. By scanning the first and second laser spots 11 and 12, therefore, first and second detection signals can be independently obtained from the BD sensor 84. The timing of BD detection is then specified by the time when a predetermined slice level is attained at the leading edge or trailing edge of a detection signal.
Since the first and second edge portions 83a and 83b are arranged parallel to the Z-axis in the coordinate system in FIG. 6, the respective light beams travel the same distance from the BD detection positions to the image writing start positions with the same delay time. This makes it possible to reduce variations in image writing start positions for the respective light beams.
In this multi-beam scanning optical system, a photosensitive device (not shown) serving as a recording medium is placed on the scanned surface 56 and is exposed by laser modulation driving based on image information. The resultant image is then visualized by a known electrophotographic process. In this manner, an image forming apparatus such as a laser printer or digital copying machine can be implemented.
If the distance from the BD sensor to an image writing start position changes depending on the dimensional precision of components and the focal length of an optical component, the delay time from BD detection to an image writing start position may be adjusted by a known method, e.g., shifting at least some of the elements constituting the synchronization detection means in a direction perpendicular to the optical axis.
The conventional multi-beam scanning optical system described above has the following problems.
(1) If a return mirror is inserted in an optical path for synchronization detection to bend the optical path in a main scanning cross-section and sub-scanning cross-section so as to make the multi-beam scanning optical system compact, jitter occurs in the main scanning direction. More specifically, if the optical path in the multi-beam scanning optical system is bent, the plane formed by a light beam scanned on the BD slit 83 for synchronization detection tilts with respect to the first and second edge portions 83a and 83b. For this reason, each light beam is obliquely scanned on a slit opening 83c. As a consequence, the time intervals at which a plurality of light beams are scanned on the slit opening 83c differ from the time intervals at which a plurality of light beams are scanned on the scanned surface, resulting in a failure to obtain a correct sync signal.
In addition, when each light beam is obliquely scanned on the slit opening 83c, the respective light beams travel different distances from the BD detection positions to the image writing start positions. If this apparatus is driven while the delay times between BD detection and image writing start positions remain the same, the image writing start positions shift from each other in a cycle of the number of light beams. As a result, an image is observed as jitter in the main scanning direction with a straight line in the sub-scanning direction becoming jagged.
To prevent this, the apparatus may be driven with different delay times being set between BD detection and image writing start positions for the respective light beams. This method, however, requires an independent delay circuit for each light beam, resulting in an increase in complexity of the overall apparatus and an increase in cost.
It is an object of the present invention to provide a multi-beam scanning optical system capable of high-speed printing operation, in which a slit member forming one element of a synchronization detection means for controlling the timing at a scanning start position on a scanned surface is set in a proper direction to prevent jitter in the main scanning direction and attain an improvement in image quality without requiring any complicated optical path arrangement, and an image forming apparatus using the multi-beam scanning optical system.
According to one aspect of the invention a multi-beam scanning optical system comprises light source means having a plurality of light-emission points incident optical means for guiding a plurality of light beams emitted from said light source means to deflection means scanning optical means for forming the plurality of light beams reflected/deflected by the deflection means into images on a scanned surface, and synchronization detection means in which a part of the plurality of light beams from the deflection means are reflected at a predetermined angle in a sub-scanning cross-section by reflection means to be scanned on a surface of a slit member and to be guided to a surface of a synchronization detection element via the slit member, and a timing at a scanning start position on the scanned surface is controlled by using a signal from the synchronization detection element,
wherein the slit member is positioned such that a plurality of light beams scanned on a surface of a slit opening are substantially vertically scanned on the slit opening.
According to further aspect of the invention, the synchronization detection means controls a timing at a scanning start position on the scanned surface in a cycle of the plurality of light beams emitted from said light source means.
According to further aspect of the invention, a longitudinal direction of the slit opening of the slit member is nonparallel to a rotational axis direction of the deflection means.
According to further aspect of the invention, a plurality of light-emission points are spaced apart from each other in at least the main scanning direction.
According to further aspect of the invention, an image forming apparatus comprises the multi-beam scanning optical system set out in the foregoing, a photosensitive member disposed on the scanned surface, a developing unit for developing an electrostatic latent image formed on said photosensitive member by a light beam scanned by said multi-beam scanning optical system into a toner image, a transfer unit for transferring the developed toner image onto a transfer medium, and a fixing unit for fixing the transferred toner image on the transfer medium.
According to further aspect of the invention, an image forming apparatus comprises the multi-beam scanning optical system set out in the foregoing, and a printer controller for converting code data input from an external device into an image signal and inputting the signal to said multi-beam scanning optical system.
According to further aspect of the invention, a multi-beam scanning optical system comprises light source means having a plurality of light-emission points, incident optical means for guiding a plurality of light beams emitted from said light source means to deflection means, scanning optical means for forming the plurality of light beams reflected/deflected by the deflection means into images on a scanned surface, and synchronization detection means in which a part of the plurality of light beams from the deflection means are reflected at a predetermined angle in a sub-scanning cross-section by reflection means to be scanned on a surface of a slit member and to be guided to a surface of a synchronization detection element via the slit member, and a timing at a scanning start position on the scanned surface is controlled by using a signal from the synchronization detection element,
wherein the slit member is positioned such that a plurality of light beams scanned on a surface of a slit opening vertically cross one of edge portions of the slit opening.
According to further aspect of the invention, the synchronization detection means controls a timing at a scanning start position on the scanned surface in a cycle of the plurality of light beams emitted from said light source means.
According to further aspect of the invention, a longitudinal direction of the slit opening of the slit member is nonparallel to a rotational axis direction of the deflection means.
According to further aspect of the invention, the plurality of light-emission points are spaced apart from each other in at least the main scanning direction.
According to further aspect of the invention, an image forming apparatus comprises the multi-beam scanning optical system set out in the foregoing, a photosensitive member disposed on the scanned surface, a developing unit for developing an electrostatic latent image formed on said photosensitive member by a light beam scanned by said multi-beam scanning optical system into a toner image, a transfer unit for transferring the developed toner image onto a transfer medium, and a fixing unit for fixing the transferred toner image on the transfer medium.
According to further aspect of the invention, an image forming apparatus comprises the multi-beam scanning optical system set out in the foregoing, and a printer controller for converting code data input from an external device into an image signal and inputting the signal to said multi-beam scanning optical system.
According to further aspect of the invention, a multi-beam scanning optical system comprises light source means having a plurality of light-emission points, incident optical means for guiding a plurality of light beams emitted from said light source means to deflection means, scanning optical means for forming the plurality of light beams reflected/deflected by the deflection means into images on a scanned surface, and synchronization detection means in which a part of the plurality of light beams from the deflection means are reflected at a predetermined angle in a sub-scanning cross-section by reflection means to be scanned on a surface of a slit member and to be guided to a surface of a synchronization detection element via the slit member, and a timing at a scanning start position on the scanned surface is controlled by using a signal from the synchronization detection element,
wherein a longitudinal direction of the slit opening of the slit member is nonparallel to a rotational axis direction of the deflection means, and the slit member is positioned such that a plurality of light beams scanned on a surface of a slit opening are substantially vertically scanned in the longitudinal direction of the slit opening.
According to further aspect of the invention, the synchronization detection means controls a timing at a scanning start position on the scanned surface in a cycle of the plurality of light beams emitted from said light source means.
According to further aspect of the invention, the slit member is positioned to be vertical or substantially vertical to a plane formed by a plurality of light beams scanned on the surface of the slit member.
According to further aspect of the invention, the plurality of light-emission points are spaced apart from each other in at least the main scanning direction.
According to further aspect of the invention, an image forming apparatus comprises the multi-beam scanning optical system set out in the foregoing, a photosensitive member disposed on the scanned surface, a developing unit for developing an electrostatic latent image formed on said photosensitive member by a light beam scanned by said multi-beam scanning optical system into a toner image, a transfer unit for transferring the developed toner image onto a transfer medium, a fixing unit for fixing the transferred toner image on the transfer medium.
According to further aspect of the invention, an image forming apparatus comprises the multi-beam scanning optical system set out in the foregoing, and a printer controller for converting code data input from an external device into an image signal and inputting the signal to said multi-beam scanning optical system.